The use of Orthogonal Frequency Division Multiplexing (“OFDM”) technology is ever increasing within wireless applications such as cellular and Personal Communication Systems (“PCS”) due to its reliability and high spectral efficiency. OFDM has a high tolerance to multipath signals and is spectrally efficient which makes it a good choice for wireless communication systems. OFDM has gained considerable interest in diverse digital communication applications due to its favorable properties like high spectral efficiency, robustness to channel fading, immunity to impulse interference, uniform average spectral density, and capability of handling very strong echoes. OFDM technology is now used in many new broadband communication schemes and many other wireless communication systems.
More specifically, OFDM is a special form of multicarrier modulation that uses Digital Signal Processor (“DSP”) algorithms such as Inverse Fast Fourier Transform (“IFFT”) to generate waveforms that are mutually orthogonal and Fast Fourier Transform (“FFT”) for demodulation operations.
However, there are some concerns with regard to OFDM. Such concerns include high Peak-to-Average Power Ratio (“PAPR”) and frequency offset. A high PAPR causes saturation in power amplifiers, leading to intermodulation products among the subcarriers and disturbances of out-of-band energy. Therefore, it is desirable to reduce the PAPR. In order to meet the out-of-band emissions requirements, a power amplifier and other components with this high PAPR input are required to provide good linearity in a large dynamic range. This power requirement makes the power amplifier one of the most expensive components within the communication system. The high PAPR also means that the power amplifier operation has low power efficiency that reduces battery life for related mobile stations. An elevated PAPR for infrastructure amplifiers increases power consumption and heat generation, compromising system reliability and limiting deployment options due to system cooling requirements.
An OFDM signal exhibits a high PAPR because the independent phases of the sub-carriers mean that the sub-carrier signals may often combine constructively allowing the peak of the signal to be up to N times the average power (where N is the number of sub-carriers). These large peaks increase the amount of intermodulation distortion resulting in an increase in the error rate. The average signal power must be kept low in order to prevent transmitter amplifier gain limiting. Minimizing the PAPR allows a higher average power to be transmitted for a fixed peak power, improving the overall signal to noise ratio at the receiver. It is therefore desirable to reduce or otherwise minimize the PAPR.
Traditionally, in order to handle a high PAPR, a system uses a linear signal chain. Any non-linearity in the signal chain will cause intermodulation distortion and degrades signal quality. The linearity requirement is demanding, especially for transmitter RF output circuitry where amplifiers are often designed to be non-linear in order to minimize power consumption.
Prior PAPR reduction methods may be classified into two groups including Constellation Shaping (“CS”), e.g., distortionless or active constellation expansion, and Tone Reservation (“TR”). With CS methods, the modulation constellation is changed such that the obtained PAPR is less than the required value with the satisfied channel error criteria. With TR methods, the reserved tones are assigned with such values that the obtained PAPR is less than the required value with the satisfied channel error criteria. In the tone reservation method, the idea is to reserve a small set of tones, or sub-carriers, for PAPR reduction. The amount of PAPR reduction depends on the number of reserved tones, their locations within the frequency vector, and the amount of complexity. Other methods of reducing PAPR are also possible but they affect signal quality or Error-Vector Magnitude (“EVM”). One such method is disclosed in United States Patent Publication No. 2007/0140101, to Guo et al., published Jun. 21, 2007 and entitled “System and Method for Reducing Peak-to-Average Power Ratio in Orthogonal Frequency Division Multiplexing Signals using Reserved Spectrum,” the entire teachings of which are hereby incorporated by reference.
In practical OFDM systems, a small amount of peak clipping is allowed to limit the PAPR in a tradeoff against linearity and power consumption. However, the transmitter output filter, which is required to reduce out-of-band spurs to legal levels, has the tendency to regrow peak levels that were clipped, thus clipping alone has not been an effective way to reduce PAPR. One method of TR PAPR reduction clips the peak signal and limits signal re-growth by distributing the excess clipped energy among the known reserved sub-carriers while preventing this distortion or noise to affect any active sub-carriers, i.e., sub-carriers carrying user data. However, at any given time, not every “active” or non-reserved sub-carrier is actually carrying data or would be adversely affected by the addition of a small amount of noise. Additionally, although having a high number of reserved sub-carriers for energy distribution aids in reducing the overall PAPR, the result is that there are fewer sub-carriers available to carry data. Thus, prior TR methods also decrease the potential system capacity.
Therefore, what is needed is a system and method for reducing the peak-to-average power ratio of OFDM communication systems by adaptively distributing excess clipped energy among reserved and active sub-carriers.